The present invention provides a pharmaceutical composition for treating cancer, the composition comprising as the active ingredient a compound represented by general formula I: C-[-CH2-O-(-CH2CH2-O-)n-X1-CHR1-O-CO-NR2-CH2-X2]4 (I) (where X1-CHR1-O-CO-NR2 is a linker, X1 is a spacer, R1 is an optionally substituted C1-4 alkyl, R2 is a phenyl or substituted phenyl group, X2 is SN-38, and n is an integer of 200 to 1,000). The composition is characterized by the following: that the composition can be used in a subject having cancer at a dosage regimen that calls for parenteral administration of a single dose of approximately 0.01 to 11 µmol/kg body weight at a frequency of once a week every other week for at least two administrations； and that the composition does not have irinotecan-like adverse reactions but does impart the same or better cancer proliferation-inhibiting activity as irinotecan at a dose that is 1/10 (molar ratio) or less that of irinotecan. Also provided is a method for treating cancer.
